1. Field of the Invention
This invention relates to sheet feed devices for use mainly in copiers, laser beam printers, facsimile devices, dot printers and ink-jet printers and other image forming apparatuses for producing images on sheets.
2. Description of the Related Art
Conventionally, the sheet feed device for the imaging forming apparatus described above has been made to operate with selection of interchangeable sheet cassettes of different size or paper quality according to necessity. As examples of the cassette mention may be made of one having a separation pawl as disclosed in U.S. Pat. No. 4,017,181, or another one without it as shown in Japanese Laid-Open Utility Model Application No. Sho 61-200838.
The prior known sheet feed device of the cassette without a type using the cassette without a separation pawl-less cassette will be described below.
In the past, an example of the sheet feed device for the copier or like image forming apparatus is shown generally at 1', having a feed roller 3 in cooperation with the releasably attached cassette 5', as shown in FIG. 6(a). Sheets S . . . stacked in the cassette 5' are fed one at a time in separation from the others by the feed roller 3 and a separation pad 6 positioned in confronting relation to the feed roller 3. Also, the cassette 5 is provided with a middle plate 7 bearing many sheets S . . . at once. The middle plate 7 is raised upward by a lifter 9 so that the top one of the sheets S . . . is pressed on the feed roller 3. Thus, the sheets S . . . are smoothly sent out in sequence.
By the way, because the remaining sheets S . . . in the cassette 5' continue being raised by the lifter 9, a certain number of sheets S.sub.1, which are counted from the top, stand out from the upper edge of the front side wall 5'a of the cassette 5'. Therefore, when the cassette 5' is taken out of a main body 2 of the sheet feed device 1', the upper sheets S.sub.1 are left behind in the main body 2 by the action of inertia or the like, and they later slip down from the cassette 5' (see FIGS. 6(b) and 6(c)).
With another type of cassette having the separation pawl, because the leading side edge of the sheet S is retained by the separation pawl, such a problem scarcely arises. But, if, as successive two or more of the sheets are partly overlapped with each other in going out, there remain a number of sheets overrunning the separation pawl, these remaining sheets will occasionally slip down from the cassette when it is taken out.